University assigned GUIDELINES for class requirements. For example, a class like calc 2 will have calc 1 as its pre-req. Pre-reqs for a class should be taken before the class, co-reqs can be taken together with a class.
Take note that these are university guidelines, and vary in quality from class to class. Some pre-reqs make sense and should be followed (Calc1-calc2), but some just make no sense (E.g, Engineering Multi is a pre-req for Engineering LinAlg, but there is no calc at all in LinAlg).
For the most part, requirements are not enforced, and you can enroll in class without the requirements. Consult your seniors who have taken the courses, or your advisor when you get one.